Both wind farms and associated electrical grids suffer faults due to storms, demand fluctuations, and the like. Moreover wind farms and the associated electrical grids require periodic maintenance. Therefore, wind turbine generators (WTGs) in wind farms must frequently start-up (i.e., start supplying power) without any external source of power, such as an electrical grid. However, preparing a WTG to start up typically requires substantial energy. For example, before a WTG can start supplying power, certain start-up procedures are often necessary, such as yawing the WTG upwind, pre-heating certain components, and pre-heating fluids, such as oil and water.
Furthermore, certain WTGs, particularly off-shore WTGs, supply power to an electrical grid through a high voltage DC bus (i.e., DC-connected WTGs). Such DC-connected WTGs typically have a power conversion stage that converts AC power output by a generator in the WTG to DC power that is supplied to the DC bus. A large capacitor in this power conversion stage (i.e., a DC-link capacitor) must be pre-charged before the WTG is able to supply power to the electrical grid. Using power supplied by the electrical grid to pre-charge the DC-link capacitor is often impractical because of the large voltage difference between the DC bus (e.g., about 45 kV to about 66 kV) and the DC-link capacitor voltage (e.g., about 5 kV). Therefore, even when electrical grid power is available for use during start-up, DC-connected WTGs cannot readily use this power to pre-charge their DC-link capacitors.
U.S. Pat. No. 8,000,840 (the '840 patent) describes a method of starting up at least a part of a wind power plant connected to an external grid substantially without any energy delivered from the external grid. The wind power plant has a plurality of wind turbines and at least one power source connected to at least one of the wind turbines in order to start the wind turbine while isolated from the remaining wind turbines. The started wind turbine may then supply power in order to start further wind turbines. According to the '840 patent, the at least one power source used to start the initially started wind turbine could be a small gas turbine, a diesel generator, a battery, or a fuel cell.
However, unlike the wind turbines, some of the power sources identified by the '840 patent are non-renewable and/or harmful to the environment. Therefore, such auxiliary power sources counteract certain environmental benefits that are a primary reason for using wind farms in the first place. Moreover, after an electrical grid outage there is usually some uncertainty as to when the electrical grid will be ready to receive and distribute power supplied by a wind farm. Thus, even if a wind farm, such as that of the '840 patent, is able to start up on its own without power from the electrical grid, if the electrical grid is not available to distribute power that the wind farm generates, preparing the wind farm for start-up will not be cost effective until the electrical grid is available anyway. In addition, because the start-up procedures can last a long time, particularly a pre-heating stage, which can last more than half a day or even up to 24 hours depending on the surrounding temperature, the wind farm will not start supplying power to the electrical grid until well after the electrical grid becomes available to receive and distribute the power. Consequently, the wind farm's power production factor will be undesirably limited.